As a coagulation reaction tank of this class, Japanese Utility Model Public Disclosure SHO 62(1987)-48405 discloses one comprising a cylindrical tank, rotary stirring means disposed upright in the central part of the cylindrical tank, provided with stirring vanes, and adapted to stir sludge supplied thereto with a coagulant and convert the sludge into floc, and a filtration tank disposed concentrically in the upper internal part of above the cylindrical tank and adapted to draw in through the bottom thereof the liquid from within the cylindrical tank in the form of filtrate and discharge the filtrate out of the cylindrical tank (first conventional technique). Japanese Utility Model Public Disclosure SHO 62(1987)-118602 discloses a coagulation reaction tank which comprises a cylindrical tank and a filtration tank disposed concentrically in the upper internal part of the cylindrical tank and adapted to draw in the liquid from within the cylindrical tank in the form of filtrate through slits formed in a plurality of horizontal stages in the outer peripheral wall of the filtration tank and discharge the filtrate out of the cylindrical tank (second conventional technique).
Further, a coagulation reaction tank using a cylindrical tank and causing the liquid within the tank to be taken out in the form of filtrate from the upper part of the tank wall is disclosed in Japanese Utility Model Public Disclosure SHO 61(1986)-810 and SHO 61(1986)-811 (third conventional technique).
The conventional techniques are invariably capable of discharging filtrate containing substantially no floc from within a filtration tank and releasing floc of ample strength from a cylindrical tank. In the case of the first conventional technique, since the bottom of the filtration tank is formed with a punched plate or a netting, minute particles of the floc or minute fibrous particles entrained by the sludge clog the holes in the punched plate or the meshes of the netting.
As a solution, therefore, the rotary vanes of the stirring means are provided with a wiper such as brushes or rubber plates adapted to be rotated and rubbed against the upper and lower surfaces of the bottom of the filtration tank so as to sweep the bottom of the filtration tank. The wiper, however, fails to manifest a fully satisfactory cleaning effect because the fibrous particles entangle on the brushes or rubber plates. In the case of the second conventional technique, the minute particles of floc or the minute fibrous particles entrained by the sludge clog the multiplicity of horizontal slits in the outer peripheral wall of the filtration tank.
As a solution, the rotary shaft of the stirring means is provided with brushes attached radially thereto and adapted to rub the inner surface of aforementioned outer peripheral wall and prevent the slits from the clogging. The brushes, however, fail to manifest fully satisfactory cleaning effect because the fibrous particles entangle on the brushes
In the case of the third conventional technique, since vertical slits are formed in the tank wall, the fibrous particles in the sludge catch in the slits As a solution, a rotating brush is brought into contact with the slits from outside the tank to clean the slits. The rotary brush, however, fails to manifest a fully satisfactory cleaning effect because the fibrous particles similarly entangle on the brush.